1. Field of the Invention
The present invention pertains to methods used to eliminate or, at least, to greatly attentuate spurious echos which occur in electro-acoustic delay lines that use bulk waves.
2. Description of the Prior Art
Prior art methods for delaying electrical signals include the use of an electro-acoustic delay line in which the electrical signals are first converted into sonic or ultra-sonic sound signals. These signals are propagated in the line at a speed which is far smaller than that of light, thus giving substantial delays in a small volume. After undergoing the desired delay, the acoustic signals are then reconverted into electrical signals.
This kind of electro-acoustic delay line of the reflection type, for example, is shown in FIG. 1. It comprises a propagation medium 10 made of corundum for example. This medium generally takes the form of a cylinder-shaped elongated rod for example. An electro-acoustic transducer 11 formed of a thin film of piezoelectric material fitted with metallic electrodes joined to connections 12 is applied to the terminal side 15 of this rod. Under the effect of an electrical signal applied to these connections, this transducer excites a sound wave in the rod and this sound wave is propagated to the other terminal side 14 of the rod along an acoustic beam 13 with a section which is substantially constant and equal to the area of the transducer. This sound wave is reflected on the surface 14 and returns to the surface 15 where it excites the transducer 11 to give, at the connections 12, an electrical signal similar to the initial signal barring the attenuation due to the transmission. A line of this type is, of course, unusable for delaying pulses which are distinct from one another since the input transducer also acts as an output transducer. Hence, a pulse of this type is delayed by a duration t=2L/v where L is the length of the rod between its sides 15 and 14 and v is the speed of the sound waves in the medium 10.
However, it is not possible to make the transducer perfectly adapted to the propagating medium, and a portion of the acoustic energy received by this transducer is again reflected so that it sets off again towards the side 14. It is reflected in turn from this side 14 and returns to the transducer and excites it, producing a spurious echo which has a delay t with respect to the main signal formed by the useful echo.
The amplitude of this spurious echo, known as a double-path echo or a double-transit echo must be reduced to the minimum.
The same problem arises in transmission lines comprising two transducers where, for example, one is a transmitting transducer on the side 15 and the other a receiving transducer on the side 14. These transmission lines are used essentially to delay the continuous signals for which the transmitter must be properly separated from the receiver. In this case, the spurious echo is the one that returns to the side 14 after being reflected successively from this side 14 and the side 15. This is a triple-path or triple-transit echo.
One prior art method for eliminating these echos is to slightly tilt the terminal sides of the rod 10 with respect to the axis of propagation. For the useful path, the acoustic beam is just slightly offset with respect to the axis of the rod, but for the spurious path the acoustic beam no longer encounters the receiving transducer and preferably reaches the wall of the rod at a place where an absorbent product is fixed. A technique of this type is described, for example, by CROFUT in MICROWAVE JOURNAL 1967, Vol. 10, pp 65 to 72.
Prior art methods as described in the French patents Nos. 2.522.452 and 2.523.784 also include the making of electric acoustic delay lines with at least one terminal side having a concave shape by which the sound waves can be focused to reduce losses. In these delay lines, the sound waves move to and fro several times to obtain substantial delays of several tens of microseconds for example, and the spurious echos are reduced by a geometrical arrangement and by respective positions of the transducers and the focusing terminal sides which make it possible to divert the spurious echos.
These various methods are difficult to apply and complicate the manufacturing of a device which has to be simple and sturdy.
3. Summary of the Invention
According to the invention, to eliminate the spurious echos, path differences are created in the acoustic beam to provoke phase shifts such that the spurious echo is destroyed by interference as far as double transit echos and, as the case may be, triple transit echos are concerned.
For this, the acoustic beam sub-divided into a set of beams having the required path difference by making a topographical array on at least one of the surfaces which encounter the beam, the said array being made on the elements from which the beams are reflected with distinct delays.
Although the phase opposition is absolutely perfect for only one frequency, the attenuation of the spurious echos is still substantial in a band of about 20% to 30% around this optimum frequency.